JPS6025581A - Disposal of hexavalent chromium-containing coating material - Google Patents

Abstract

PURPOSE:To enable hexavalent chromium contained in a coating material to be discarded or be recovered as a valuable substance, by reducing hexavalent chromium to harmless trivalent chromium. CONSTITUTION:A hexavelent chromium-containing material is incinerated in an incinerator by adding appropriate combustion aids thereto. Then, the incinerated ash of the coating material is placed in a reducing device, water is added thereto to form a slurry, and after acidifying the slurry by adding a pH-adjusting agent thereto, sodium hydrogen sulfite, ferrous sulfate or sodium sulfite is added thereto as a reducing agent, and the resultant mixture is agitated. After reduction is completed, the slurry is neutralized, and is dehydrated to form a dehydrated cake, which can be disposed of as a waste material for use in reclamation or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for rendering hexavalent chromium-containing paint harmless and disposing of it.

Conventionally, hexavalent chromium-containing paints have been disposed of by force by adding solidifying agents such as cement or asphalt and disposing of the solidified material in landfills or in the ocean. However, the solidified material may cause damage and erosion of the solidifying agent due to aging or impact, and hexavalent chromium may be eluted and re-contaminate the environment. Further, the above-mentioned disposal methods such as solidification, landfill, and ocean dumping have problems such as requiring a large amount of cost and equipment.

The present invention solves the above problems and provides a method that can reduce hexavalent chromium in paint to trivalent chromium, render it harmless, and dispose of it or recover it as a valuable resource. A method for disposing of a hexavalent chromium-containing paint is characterized in that the hexavalent chromium-containing paint is incinerated and then the hexavalent chromium is reduced to trivalent chromium using a reducing agent.

The above-mentioned incineration process can be carried out using a normal incinerator, but if necessary, a suitable combustion aid can be added to speed up the combustion. Further, a dust collector can be provided in the middle of the combustion exhaust and gas exhaust path to remove hexavalent chromium dust in the exhaust gas.

The above reduction can be carried out by putting the incineration ash of the hexavalent chromium-containing paint into a reduction device, adding water to make a slurry, adding a pH adjuster to make it acidic, and then adding a reducing agent and stirring. can. As the reduction device, any device such as a drum rotation type, a mixer, or a batch type may be used. Also PH
The amount and type of the adjusting agent to be added can be appropriately selected in consideration of the PH of the incinerated ash slurry, the PH range in which the reducing agent can perform the reducing action, and the like. For example, if sodium bisulfite is used as a reducing agent, sulfuric acid is used to reduce the pH
, 5 or less.

As the reducing agent, sodium bisulfite, ferrous sulfate, sodium sulfite, etc. can be used.

After the reduction reaction is completed, the slurry is neutralized by adding a suitable neutralizing agent, such as slaked lime.

The amount and type of neutralizing agent to be added can be appropriately determined depending on the amount of slurry, pH, etc.

The slurry is then dewatered according to conventional methods to form a dewatered cake, which can be disposed of, such as in a landfill.

Note that the order in which the water, PH adjuster, reducing agent, and incinerated ash are introduced into the reduction device is not limited to the above-mentioned order.
This can also be done in the following order: first put water into the reducing device, then add the PH adjuster and reducing agent, mix [7], and then add the incinerated ash.

Further, when gas is generated as the reduction reaction progresses, it is desirable to provide a gas outlet at an appropriate location in the reduction device to allow the gas to be discharged.

As described above, since the present invention utilizes a chemical reaction, a treatment time of several hours is sufficient, which is more advantageous than in-place treatment, which requires a long treatment time. In addition, the equipment required for treatment may be smaller than that required for solidification treatment, etc., and landfill costs can be reduced, making it possible to reduce treatment costs.

Example 1 Waste hexavalent chromium-containing paint is incinerated to produce 80 kg of incinerated ash.
I got it. The hexavalent chromium elution concentration of this incineration ash is 85 ppm.
Met.

Next, put 300 ml of water into the drum rotary reduction device, add 5 ml of sodium bisulfite to 9 ml of water. After adding and mixing 9 kg of sulfuric acid, 80 kg of the above incinerated ash was added and stirred by rotating the drum. The pH of the reaction mixture was below Z5. As the reaction progressed, sulfur dioxide gas was generated and was released from a degassing pulp provided in the reduction device. After standing still for about 5 hours, 9/201 water was added to slaked lime milk 8 to neutralize it.

The concentration of hexavalent chromium eluted from the incinerated ash after treatment was measured and found to be Oppm.

Example 2 Waste hexavalent chromium-containing paint is incinerated and incinerated ash 64 to 9
I got it. The hexavalent chromium elution concentration of this incineration ash is as pp
It was m.

Next, put 200 ml of water into the drum rotary reduction device, and add 4 ki 9 ml of bisulfite soda. After adding 7 ml of sulfuric acid and mixing,
The above incineration ash was added and stirred by rotating the drum. As the reaction progressed, sulfur dioxide gas was generated, which was released from a degassing pulp provided in the reduction device. After leaving the mixture still for about 5 hours, 7 kg/20 of slaked lime milk was added to one water to neutralize it.

The concentration of hexavalent chromium eluted from the incinerated ash after treatment was measured and found to be Oppm.

Claims (2)

[Claims] (1) A method for disposing of paint containing hexavalent chromium, which comprises incinerating the paint containing hexavalent chromium and then reducing the hexavalent chromium to trivalent chromium using a reducing agent. (2) The reducing agent is sodium bisulfite or ferrous sulfate. The method according to claim 1, wherein the sodium sulfite is sodium sulfite.